The essential amino acid tryptophan (TRP) is a precursor for the synthesis of nicotinamide adenine dinucleotide (NAD+) and serotonin. TRP is metabolized by two pathways the kynurenine (KYN) pathway resulting in NAD and the methoxyindole pathway which generates serotonin and melatonin. The KYN pathway is important in controlling immune function in the setting of inflammation, since kynurenine metabolites exhibit immune suppressive activity. Furthermore, the downstream product quinolinic acid exhibits neurotoxic properties and is likely involved in neuronal injury in a variety of central nervous system diseases. Excess tryptophan catabolism likely depletes the necessary tryptophan needed for the biosynthesis of serotonin, potentially leading to depression, and melatonin, potentially leading to sleep disorders. An increased kynurenine to tryptophan ratio has been observed and implicated in the pathogenesis of several disease states including cardiovascular disease, Alzheimer's disease, amyotrophic lateral sclerosis, Huntington's disease, Parkinson's disease, stroke and epilepsy (Central Nervous System Agents in Medicinal Chemistry, 2007, 7, 45-56), malaria, cancer, depression, schizophrenia, obesity, eating disorders, metabolic syndrome, insulin resistance, diabetes, osteoporosis, rheumatoid arthritis, migraine, systemic lupus erythematosus, and HIV infection. In HIV infection, increased tryptophan catabolism has been implicated in immunopathogenesis as well as comorbid conditions associated with HIV/AIDS. Notably, depression and sleep disorders are common comorbidities in HIV infection. The metabolism of tryptophan has critical relevance in pregnancy (survival of the allogeneic conceptus/fetus), transplant biology, cancer therapeutics, as well as normal immunologic control (Mellor and Munn, Immunol Today. 1999, 20:469-473, Grohmann et al., Trends Immunol. 2003, 24:242-248).
The initial, rate-limiting step in the kynurenine pathway is performed by heme-containing oxidoreductase enzymes, including indoleamine 2,3-dioxygenase (IDO). Increased production of the enzyme IDO is exhibited in cancers, as a consequence of inflammatory signals such as interferons, NFkB activation, and TNF alpha, as well as the combined stimulus of LPS. With IDO promoting the rate limiting step in tryptophan breakdown, this process leads to reduced local levels of tryptophan as well as increased kynurenine metabolites (Prodinger et al., J Leukoc Biol. 2016, 99:583-594) that both contribute to immune suppression and HIV associated pathogenesis. The mechanism whereby regulatory myeloid cells degrade tryptophan as well as arginine (via arginase 1 in M2 macrophages) results in the accumulation of uncharged tRNAs in T cells, activation of GCN2 kinase in naïve and regulatory T cells, resulting in immune suppression via inhibition of effector T cell function (Boasso and Shearer, Curr Drug Metab. 2007, 8:217-223; Mellor and Munn, Nat Rev Immunol. 2008, 8:74-80; Barth and Raghuraman, Crit Rev Microbiol. 2014, 40:360-368; Schmidt and Schultze, Front Immunol. 2014, 5:384). The inhibition of tryptophan catabolism is therefore an important target for immune modulation; IDO is the subject of intense investigation in clinical trials for cancer (Vacchelli et al., Oncoimmunology, 2014, 3:e957994).
Although NAD+ can be synthesized from dietary precursors (such as niacin) through salvage pathways, TRP catabolism is required for the de novo synthesis of NAD+. NAD+ is involved in virtually all biological processes, including energy transfer, and is the active subject of investigation for therapeutics in many diseases which include Parkinson's disease, Alzheimer's disease, cardiovascular disease as well as aging (Mouchiroud et al., Crit Rev Biochem Mol Biol. 2013, 48:397-408). NAD+ is required for the action of Sirtuin deacetylases, where their action is dependent upon the availability of NAD+. Strategies that either increase NAD+ production via niacin related salvage pathways and/or strategies that prevent NAD+ consumption/degradation by downstream pathways are the subject of intense interest as therapeutic strategies for age related diseases.
HIV infection is associated with increased TRP metabolism as determined by elevated kynurenine to tryptophan ratio, which often incompletely resolves in the context of antiviral therapy. Continued elevations in the kynurenine to tryptophan ratio is associated with CD4+ T cell decline, more rapid decline in CD4/CD8 ratios, and the development of comorbidities associated with HIV infection including cardiovascular disease, HIV associated neurocognitive disorder (HAND), depression and sleep disorders.
HIV enters the central nervous system within the first weeks of infection. A mature CD14+ CD16+ monocyte subset may contribute to HAND by transporting virus into the CNS, by serving as a target for HIV infection in the CNS, contributing to inflammation and vascular injury within blood vessels and CNS compartments, and promoting neuro-inflammatory injury. This subset constitutes only 5-10% of peripheral blood monocytes in healthy seronegative individuals but their percentage increases in HIV infected people. CD16+ monocytes in circulation correlate with HAND, cardiovascular disease, as well as other disorders involving monocyte macrophage activation.
HIV infection can be effectively suppressed by current anti-retroviral therapy regimens, however, the virus remaining is recalcitrant to eradication due to the long-lived nature of latently infected cells, as well as low levels of ongoing virus replication. There are also significant side effects and economic costs of long-term antiviral therapies. Up to now latency reversing strategies have been insufficiently successful and new paradigms are needed to eliminate latent and persistent infection.
TRP catabolism is also important chronic hepatitis C infection in HIV/Hepatitis C co-infection (Jenabian et al., J Acquir Immune Defic Syndr. 2016 Mar. 1; 71(3):254-62), as well as in hepatitis C infection alone (Asghar et al., Exp Ther Med. 2015 March; 9(3):901-904). Typtophan catabolism as determined by IDO activity and kynurenine metabolites correlate liver disease pathogenesis, where liver disease is particularly difficult to treat in HIV/HCV coinfection, despite treatment with cART and ribivarin and pegilated interferon for HIV and HCV respectively. While HCV appears to be more difficult to treat in HIV co-infected patients, TRP catabolism remains elevated in co-infected patients using interferon containing treatment strategies, despite sustained virological response (SRV) to HCV (even after 6 months of SRV), relative to monoinfected patients. Even in HCV and HIV/HCV co-infected patients treated with interferon free regimens, cirrhosis and inadequate SRV remains a significant problem for a large fraction of patients (Nicolini et al., Eur J Gastroenterol Hepatol. 2016 January; 28(1):37-41).
Liver disease is promoted by excess tryptophan catabolism in HCV/HIV patients, despite effective cART treatment and/or HCV treatment. Targeting IDO has been proposed, as a therapeutic in this setting, however, targeting IDO alone may not be sufficient to suppress inflammation.
There is a need in the art for improved methods for modulating pathways associated with tryptophan and NAD+ metabolism for treating diseases, particularly to combine strategies to support NAD+ production, reduce NAD+ hydrolysis, and increase the activity of Sirtuins. The present invention satisfies this unmet need.